Intercarrier method: Difference between revisions

Revision as of 06:05, 28 July 2010

Revision as of 18:04, 20 August 2010

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{{articlemultiple issues|article=yes|cleanup=November 2009|unreferenced=November 2009|wikify=November 2009}}

The '''intercarrier method''' is a system in television that reduces the cost of transmitters and receiver sets by processing audio and video signals together and minimizing the number of separate stages for audio and video signals.

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{{main|Television transmitter}}

In TV transmitters, both AF and VF [[modulate]] [[intermediate frequency]] (IF) carriers. (The frequency difference between the two carriers is 4.5  MHz. in [[system M]] and 5.5  MHz. in [[system B|system B/G]]) Then the modulated IF signals are added either at the output of the [[Modulator|vision modulator]] or at the output of the vestigial sideband stage. In both cases, the added signals are low level signals and no special combining circuitry is required.

[[Frequency conversion]] and [[amplification]] is common. So a [[Electronic mixer|frequency convertorconverter]] (or a mixer) and a series of amplifiers for aural signal as well as an output [[combiner]] are spared, which reduce the cost of the transmitter and electricity consumption of the amplifiers considerably. Although a [[Band-stop filter|notch filter]] to suppress the intermodulation products is used at the output of the intercarrier transmitter, the cost of the notch filter is not comparable to the cost of extra amplifiers and the output combiner. (See the subsection '''Intermodulation products''' below)

== Intercarrier as used in receiver sets ==

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{{main|Television set}}

In TV receivers, the received radio frequency signal is converted to IF in tuner and then demodulated. The output of the demodulator consists of a VF and an aural signal which is in fact an FM subcarrier modulated by AF. (The subcarrier is 5.5 &nbsp;MHz. in [[system B]] and 4.5 &nbsp;MHz. in [[system M]] ) The aural signal and the VF are separated by a simple filter. The only extra stage needed for AF (other than the [[loudspeaker]]) is an [[Demodulation|FM demodulator]].<ref> Bernard Grob,Charles E.Herndon:''Basic Television and Video Systems'', Glencoe, McGraw Hill, New York, 6th ed, 1999, p 469.</ref> The intercarrier receiver system makes for easier tuning of a TV station. The viewer could fine tune such a set to get the best picture reception, and not lose sound reception. (Although AF VF signals are combined in the IF stages of the transmitters, they are separated in baseband stages of the receivers.)

== Intermodulation products and notch filters==

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{{Analogue TV transmitter topics}}

[[Category: televisionTelevision technology]]

[[Category:Broadcast engineering]]

[[Category:Broadcast transmitters]]

[[de:Intercarrierverfahren]]